OBJECTIVE To investigate the effect of anti-bovine serum B cell clone in mice on preparation of McAb. METHODS Supernatant of hybridoma cell was double screened with rSIFN-co and BSA by indirect enzyme-linked immunosorbent assay. After BSA absorption, the inhibition effects on culture supernatant of double positive hybridoma cell clone and rSIFN-co single positive hybridoma cell clones and on normal mouse serum were investigated separately. RESULTS One double positive cell clone and three rSIFN-co single positive clones were established. The reaction of rSIFN-co with the supernatant of the double positive clone was inhibited obviously on condition that the supernatant was absorbed with BSA in advance. CONCLUSIONS In the screening of hybridoma cell, there is rSIFN-co and BSA double positive hybridoma cell clone, which suggests the possible existence of innate anti-BSA B cell clone in mice; they can cause false-positive reaction in the preparation of McAb. Thus, double screening with specific antigen and BSA is beneficial.